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BMI May Be an Outdated Measurement of Health and Fitness

A recent Danish study published in PLoS ONE aims to untangle the relationship between body mass index, fitness, metabolic health, and inflammation.

Body mass index, or BMI, is a common method for determining whether an individual is obese. This method was created in the 1800s and is a ratio of a person’s body mass (kg) divided by the square of their height (m2). Thus, the following weight categories were created: underweight is categorized by a BMI under 18.5kg/m2, normal weight is between 18.5 to 25 kg/m2, overweight is between 25 to 30 kg/m2, and obese is categorized by a BMI over 30 kg/m2.

However, BMI is a contentious measurement and there is ample evidence that it is an imperfect, possibly even poor, measurement of one’s physical health and fitness. Oddly, around 30% of ‘obese’ people are considered metabolically healthy (i.e. low abdominal adiposity, low risk of metabolic comorbidities, and low levels of inflammation). Researchers from Denmark have recently published a paper in the journal PLOS ONE where they delve into this oddity and re-evaluate the exact relationship between physical fitness, metabolic health, and BMI.

Researchers gathered for this cross-sectional study from a cohort of 10,976 individuals (4,838 men and 6,338 women) that were part of The Danish National Health Examination Survey in 2007-2008. In this study, Danish citizens from 13 different municipalities were invited to complete two separate questionnaires, and a random subsample was then invited for a physical examination. At the physical exam, the researchers took a blood sample from each individual, followed by measurements of their weight, height, and waistline circumference. The physical fitness of each individual was then measured using a cycling test that recorded maximal oxygen uptake (VO2max). Each individual also reported other lifestyle information, such as alcohol consumption, smoking, education, and health.

The researchers then derived the following with the collected information:

  • each individual’s BMI using the weight and height data
  • visceral abdominal fat with waist circumference
  • C-reactive protein (hsCRP), a blood marker of inflammation low-grade inflammation

Due to concerns over physical conditions or health, a final number of 10,976 individuals were included in the complete study.

Physical Fitness and Waistline Predicted Health

The researchers found that when they controlled for age and lifestyle factors, fitness and waistline circumference were inversely associated with each other, independent of BMI values. High levels of physical fitness were also associated with lower levels of inflammation, while a higher waistline circumference was associated with increased levels of inflammation, again independently of BMI values. While the results for men and women closely resembled each other, it should be noted that men appear to be less affected by low-grade inflammation than women of similar BMI values. These results suggest that the anti-inflammatory effect of fitness in individuals may contribute to the metabolically healthy ‘obese’ that have been previously observed.

While the lifestyle habits of the individuals varied in this study, it is clear that the research team used statistical modelling to minimize the effects of such variables on the outcomes presented. It is unfortunate that it was necessary to eliminate so many individuals due to health conditions, as this somewhat limits the broader applicability of these results, however, the author acknowledges this.

This paper adds strong evidence to the fact that BMI alone is insufficient for determining the health of an individual, and that newer, more robust practices will be needed in the future. The authors suggest that waist circumference, markers of inflammation, and fitness may be better predictors of metabolic health.

Written by Michael Healy, BSc, MSc

Reference: Wedell-Neergaard, A. S., et al. (2018). “Low fitness is associated with abdominal adiposity and low-grade inflammation independent of BMI.” PLoS One 13(1): e0190645.



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